The following relates to the nuclear reactor arts, nuclear power generation arts, nuclear reactor control arts, nuclear reactor electrical power distribution arts, and related arts.
In nuclear reactor designs of the integral pressurized water reactor (integral PWR) type, a nuclear reactor core is immersed in primary coolant water at or near the bottom of a pressure vessel. In a typical design, the primary coolant is maintained in a subcooled liquid phase in a cylindrical pressure vessel that is mounted generally upright (that is, with its cylinder axis oriented vertically). A hollow cylindrical central riser is disposed concentrically inside the pressure vessel. Primary coolant flows upward through the reactor core where it is heated, rises through the central riser, discharges from the top of the central riser, and reverses direction to flow downward back toward the reactor core through a downcomer annulus.
The nuclear reactor core is built up from multiple fuel assemblies. Each fuel assembly includes a number of fuel rods. Control rods comprising neutron absorbing material are inserted into and lifted out of the reactor core to control core reactivity. The control rods are supported and guided through control rod guide tubes inside the reactor core and by guide tube frames outside the core. In the integral PWR design, at least one steam generator is located inside the pressure vessel (i.e. “integral with” the reactor), typically in the downcomer annulus, and the pressurizer is located at the top of the pressure vessel, with a steam space as the top most point of the reactor. Alternatively an external pressurizer can be used to control reactor pressure.
A set of control rods is arranged as a control rod assembly that includes the control rods connected at their upper ends with a yoke or spider, and a connecting rod extending upward from the spider. The control rod assembly is raised or lowered to move the control rods out of or into the reactor core using a control rod drive mechanism (CRDM). In a typical CRDM configuration, an electrically driven motor or magnetic assembly selectively rotates a roller nut assembly or other threaded element that engages a lead screw that in turn connects with the connecting rod of the control rod assembly. The control rods are typically also configured to “SCRAM”, by which it is meant that the control rods can be quickly released in an emergency so as to fall into the reactor core under force of gravity and quickly terminate the power-generating nuclear chain reaction. Toward this end, the roller nut assembly may be configured to be separable so as to release the control rod assembly and lead screw which then fall toward the core as a translating unit. In another configuration, the connection of the lead screw with the connecting rod is latched and SCRAM is performed by releasing the latch so that the control rod assembly falls toward the core while the lead screw remains engaged with the roller nut. See Stambaugh et al., “Control Rod Drive Mechanism for Nuclear Reactor”, U.S. Pub. No. 2010/0316177 A1 published Dec. 16, 2010 which is incorporated herein by reference in its entirety; and DeSantis, “Control Rod Drive Mechanism for Nuclear Reactor”, U.S. Pub. No. 2011/0222640 A1 published Sep. 15, 2011 which is incorporated herein by reference in its entirety.
The CRDMs are complex precision devices which typically include an electric motor requiring electrical power, and may also require hydraulic, pneumatic, or another source of power to overcome the passive SCRAM release mechanism (e.g., to hold the separable roller nut in the engaged position, or to maintain latching of the connecting rod latch) unless this is also electrically operated (e.g., an electromagnetic clamp that releases upon removal of electrical power). In existing commercial nuclear power reactors, the CRDMs are located externally, i.e. outside of the pressure vessel, typically above the vessel in PWR designs, or below the reactor in boiling water reactor (BWR) designs. An external CRDM has the advantage of accessibility for maintenance and can be powered through external electrical and hydraulic connectors. However, the requisite mechanical penetrations into the pressure vessel present safety concerns. Additionally, in compact integral PWR designs, especially those employing an integral pressurizer, it may be difficult to configure the reactor design to allow for overhead external placement of the CRDMs. Accordingly, internal CRDM designs have been developed. See U.S. Pub. No. 2010/0316177 A1 and U.S. Pub. No. 2011/0222640 A1 which are both incorporated herein by reference in their entireties.
However, a difficulty with this approach is that it entails extensive electrical (and possibly hydraulic and/or pneumatic) cabling inside the reactor pressure vessel. For example, if there are sixty nine CRDM units with three electrical cables per CRDM unit (e.g., power, position indicator, and ground), then 207 electrical cables are required for the sixty nine units. The locations of the CRDM units are substantially constrained, e.g. all CRDM units are above the reactor core in the case of a PWR, and at a distance from the core effective to allow the CRDM units to move the control rod assemblies into or out of the core. An approach for relaxing the positioning constraint is to stagger neighboring CRDM units vertically, as disclosed in U.S. Pub. No. 2011/0222640 A1. However, the space for the electrical cabling is still tight. Electrical cabling in a nuclear reactor is typically in the form of mineral insulated (MI) cables, which have limited bend radius specifications. Cabling operations such as splicing or joining cables is complex for MI cables, because the mineral insulation can be damaged by water exposure. The SCRAM function is safety-related, and so nuclear safety regulations may require shutdown of the reactor if even one CRDM unit becomes non-operative, making reliability of this extensive MI cabling of especial importance.
Disclosed herein are improvements that provide various benefits that will become apparent to the skilled artisan upon reading the following.